Locally stored thumbnails for video and YouTube uploads

- Server-side video thumbnails: ffmpeg-static grabs a frame at ~1s,
  scales it to 480px and stores it under /uploads/thumbs/ at upload
  time (direct files, remote URLs and comment attachments)
- YouTube thumbnails are downloaded once at upload time instead of
  being hotlinked from i.ytimg.com on every render; remote URL kept
  as fallback for legacy rows
- thumbUrl now covers video/audio/youtube/twitch in grid meta,
  profile uploads and notifications
- Generated thumbnail files are removed on single/bulk delete

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

server/api/uploads/index.post.ts

@@ -64,6 +64,9 @@ export default defineEventHandler(async (event) => {
       const audioMeta = media.type === 'audio'
         ? await extractAudioMeta(id, media.data, media.mimeType)
         : null
+      const videoThumb = media.type === 'video'
+        ? await extractVideoThumb(id, join(uploadDir(), fileName))
+        : null
       await db.insert(uploads).values({
         id,
         userId: user.id,
@@ -72,7 +75,7 @@ export default defineEventHandler(async (event) => {
         originalName: decodeURIComponent(basename(new URL(body.url).pathname) || body.url).slice(0, 255),
         mimeType: media.mimeType.slice(0, 100),
         size: media.data.length,
-        previewImage: audioMeta?.cover ?? null,
+        previewImage: videoThumb ?? audioMeta?.cover ?? null,
         audioMeta: audioMeta?.meta ?? null,
       })
       await ensureThreadSubscription(id, user.id)
@@ -84,7 +87,9 @@ export default defineEventHandler(async (event) => {
       type === 'link' || type === 'twitch' ? fetchLinkPreview(body.url) : null,
       twitchTarget ? fetchTwitchPreview(twitchTarget) : null,
     ])
-    const remotePreviewImage = twitchPreview?.image ?? preview?.image ?? null
+    const embedId = type === 'youtube' ? getYoutubeId(body.url) : null
+    const remotePreviewImage = twitchPreview?.image ?? preview?.image
+      ?? (embedId ? youtubeThumbUrl(embedId) : null)
     const previewImage = remotePreviewImage
       ? (await savePreviewImage(id, remotePreviewImage)) ?? remotePreviewImage
       : null
@@ -137,6 +142,9 @@ export default defineEventHandler(async (event) => {
   const audioMeta = type === 'audio'
     ? await extractAudioMeta(id, file.data, mimeType)
     : null
+  const videoThumb = type === 'video'
+    ? await extractVideoThumb(id, join(uploadDir(), fileName))
+    : null
   await db.insert(uploads).values({
     id,
     userId: user.id,
@@ -145,7 +153,7 @@ export default defineEventHandler(async (event) => {
     originalName: (file.filename ?? fileName).slice(0, 255),
     mimeType: (mimeType || 'application/octet-stream').slice(0, 100),
     size: file.data.length,
-    previewImage: audioMeta?.cover ?? null,
+    previewImage: videoThumb ?? audioMeta?.cover ?? null,
     audioMeta: audioMeta?.meta ?? null,
   })